Methods of heterogeneously polymerizing a monomer in a supercritical carbon dioxide to prepare a particulate polymer, such as emulsion polymerization, dispersion polymerization and suspension polymerization are well known.
The heterogeneous polymerization in a supercritical carbon dioxide is used for preparing particulate polymers from various monomers because of having the following advantages compared with heterogeneous polymerizations in water or organic solvents:                (1) solvent removal and drying process after polymerization can be simplified;        (2) disposal of waste solvent is unnecessary;        (3) poisonous organic solvents need not be used;        (4) remaining unreacted monomer components and harmful materials can be removed by washing; and        (5) carbon dioxide used can be collected and reused.        
The particulate polymers are used for various applications such as developers for electrophotography, inks for printings, coatings for constructions and cosmetics.
Japanese published unexamined application No. 2009-167409 discloses a method of synthesizing a colored particulate polymer from a radical polymerizable monomer under the presence of a surfactant including a perfluoroalkyl group. However, a fluorochemical surfactant used in this method is very expensive and has a problem in terms of safety. Further, particulate polymers having small molecular weight distributions (Mw/Mn=about 2 or less) cannot be obtained.
Japanese published unexamined application No. 2009-132878 discloses a method of obtaining a particulate polymer and synthesizing a polymeric surfactant in one pot at the same time using a polymeric radical polymerization initiator having an organosiloxane skeleton without separately preparing a surfactant in accordance with a monomer. However, particulate polymers having a molecular weight distribution (Mw/Mn) of 2 or less cannot be obtained, either. This does not disclose a ring-opening polymerizable monomer.
Therefore, a method of preparing a particulate polymer having a small molecular weight distribution using a ring-opening polymerizable monomer in a compressible fluid is unknown.
As a binder resin for use in a toner, thermoplastic resins from petroleum such as styrene acrylic resins and polyester resins are used. However, in recent years, in consideration of the environment, methods of using biodegradable resins having less effect thereon when wasted and formed of recyclable resources from biomass as a toner binder resin have commanded attention, and a variety of the methods are disclosed.
As one of the methods, Japanese published unexamined applications Nos. 04-179967 and 2008-262179 disclose a method of using a biodegradable microbial production aliphatic polyester as a binder resin. However, when the polyester is used as a resin for a toner, the toner has a high softening point and a high fixable temperature, and therefore this is unsuitable in terms of saving energy. As a method of lowering the fixable temperature, Japanese Patent No. 2597452 discloses a method of adding a large amount of a plant wax to the biodegradable resin to lower the softening point. The softening point of a toner can be lowered, but the wax agglutinates the toner and productivity of the toner deteriorates, and fluidity thereof also deteriorates, resulting in deterioration of feedability thereof in an image developer.
To obtain low-temperature fixability and fixing stability, Japanese published unexamined applications Nos. 2006-091278 and 2006-285150 disclose a method of using a binder resin including two resins having softening points different from each other and a biodegradable resin. In this method, the resin having a lower softening point plays a part of joining the resin having a higher softening point with the biodegradable resin, and the biodegradable resin is uniformly dispersed in a binder resin. However, when the content of the biodegradable resin is high, it is not well dispersed, resulting in deterioration of developability due to uneven chargeability and deterioration of durability. Therefore, the binder resin includes the biodegradable resin in a very small amount of 20% by weight.
Further, this is not particularly specified, but the biodegradable resin deteriorates a glass transition temperature and a heat distortion temperature because of absorbing moisture. Therefore, it has a defect of agglutination of particles or images when transported or stored in hot and humid conditions, which is unusable.
As mentioned above, the biodegradable resin has too many problems to be a main component of a binder resin of a toner, and needs further improvement in synthesizing and properties to have both heat and humidity resistance, and low-temperature fixability.
Because of these reasons, a need exists for a toner having a small molecular weight distribution, a low-temperature fixability, heat and humidity resistance, and environmental stability.